Fabrication of optical fiber by MCVD, a widely used process, includes depositing glassy material on the inside wall of a substrate glass tube, typically a nominally pure silica tube. The preform is then collapsed into a solid preform, from which fiber is drawn by a conventional technique.
Although typically being nominally pure (i.e., not intentionally doped) silica, conventional substrate tubes have relatively high (e.g., 1 dB/km) optical loss at wavelengths of interest for signal transmission, e.g., 1.54 .mu.m. It is thus necessary to ensure that substantially no optical signal power extends into the substrate-derived cladding of optical fiber. This is typically achieved through deposition on the inside of the substrate tube of a sufficient amount of cladding glass, followed by deposition thereon of the core glass.
In a widely used, commercially available single mode optical fiber for long-distance signal transmission, about 80% of the optical power is transmitted through the core of the fiber, with the remainder transmitted through the deposited cladding. The deposited cladding layer has to be thick enough to prevent extension of a significant fraction of the optical power into the substrate tube-derived (outer) cladding of the fiber. In the above referred-to optical fiber this currently is achieved by provision of enough deposited cladding glass to result in D/d .about.3.2, where D and d are, respectively, the outer diameter of the deposited cladding and the core diameter, either in the fiber or in the collapsed preform.
For obvious economic reasons, it would be highly desirable to increase the length of fiber that can be drawn from a given preform. However, straight-forward scaling-up of the preform does not appear feasible, due inter alia to the large amount of deposited glass that would be required. By way of example, a scaled-up preform that can yield 500 km of the above referred-to optical fiber would require a cladding deposit of 17 mm. It is unlikely that so much glass could be deposited on the inside of a tube in a cost-effective manner.
It will thus be necessary to make changes in fiber design and/or process parameters to make it possible to increase the length of fiber that can be drawn from a preform. This application discloses changed fiber designs and process parameters that can facilitate increased fiber yield per preform. In particular, this application discloses an optical fiber that comprises appropriately placed hydrogen gettering means.
U.S. Pat. No. 5,274,734 discloses silica-based optical fiber that further contains in its core Ge, Al and a rare earth, and that is provided with means for preventing hydrogen to reach the core of the (active) fiber during the anticipated service life of the fiber, so as to avoid or minimize changes in the loss of the fiber over the service life of the fiber. Exemplarily, the means comprise silica cladding material that contains hydrogen gettering sites, thereby reducing the indiffusion of hydrogen from the ambient into the Si--Ge--Al-rare earth-containing core of the fiber.